1. Field of the Invention
The present invention relates to a thin film device such as an oxide semiconductor thin film transistor which uses an oxide semiconductor film as an active layer. Hereinafter, the thin film transistor is referred to as “TFT”, and an oxide semiconductor composed mainly of indium (In), gallium (Ga), zinc (Zn), and oxygen (O) is referred to as “IGZO (In—Ga—Zn—O)”.
2. Description of the Related Art
Regarding a TFT in which the oxide semiconductor film is used for the active layer, the field-effect mobility is higher than that of a conventional amorphous silicon TFT for about one digit. Further, an increase in the off-current at the time of irradiating visible light is extremely small. Therefore, a high on-off ratio can be acquired. By using such characteristic, researches and developments of high-performance liquid crystal displays and organic EL displays using the oxide semiconductor TFT for the pixel drive element are being conducted broadly.
It is also a characteristic of the oxide semiconductor film to have a variety of compositions, so that researches and developments of various oxide semiconductor films such as an IGZO film, a Zn—O film, an In—Si—O film, and a Zn—Sn—O film are being conducted. While there are searches for various materials being conducted, those containing at least either indium (In) or zinc (Zn) are the mainstreams as the oxide semiconductor films which can provide a fine TFT characteristic.
Further, there is also a possibility that a fine thin film is created at a low temperature with the oxide semiconductor thin film compared to the case of using a silicon-based thin film. There are also the tests being conducted to achieve a flexible display by forming an oxide semiconductor TFT on a plastic substrate by utilizing the low-temperature deposited film.
Next, documents of conventional techniques will be described as the related techniques. Regarding the TFT using the oxide semiconductor thin film, Nomura, et al. first reported the TFT using an IGZO film as the semiconductor active layer (Nomura, et al., Nature, vol. 432, p. 488, (2004): Non-Patent Document 1). This document reports a method which employs plasma etching by using a fluorine-based gas as a patterning method of a source/drain metal electrode of an oxide semiconductor TFT.
C-J. Kim, et al. created an oxide semiconductor TFT using an IGZO film as an active layer through etching a source/drain electrode constituted with Ti by using mixed gas plasma of an Ar gas and an SF6 gas (C-J. Kim, et al., Electrochemical and Solid-State Letters. Vol. 12, H95, (2009): Non-Patent Document 2). In this Document, it is disclosed that the off-current of the TFT is increased dramatically since an oxygen deficit layer is formed on the top face (back channel of the TFT) of the IGZO thin film at the time where the source/drain electrode constituted with Ti is etched. Further, it is also disclosed that it is necessary to perform etching to remove the oxygen deficit layer by a hydrochloric acid solution in order to decrease the increased off-current.
Further, Kumomi, et al. reports an IGZO oxide semiconductor TFT using a source/drain electrode constituted with Mo (Kumomi et al., Journal of Display Technology, vol. 5, 531 (2009): Non-Patent Document 3). In this Document, it is depicted that a granular surface layer remains on a back channel surface after etching the source/drain electrode constituted with Mo by using CF4 gas plasma and that this may be a cause for the instability of the TFT characteristic. A stable characteristic is achieved by removing the surface layer through performing etching also with a hydrochloric acid solution.
T. Arai, et al. report that there is an interface layer existing in the interface between an oxide semiconductor film and a source/drain electrode (T. Arai, et al., SID' 10 Technical Digest, 69-2, (2010): Non-Patent Document 4). This Document discloses that an IGZO layer where oxygen is lost exists along with a TiOx layer in the interface between Ti as the source/drain electrode and an IGZO film as an oxide semiconductor film. However, nothing is mentioned therein regarding the surface layer of the IGZO film surface.
Etching of the source/drain electrode of the oxide semiconductor TFT is disclosed in Japanese Unexamined Patent Publication 2009-260378 (Patent Document 1). This Document discloses a manufacturing method which performs etching of an oxide semiconductor TFT channel with a gas containing fluorine or chlorine. However, nothing is mentioned therein regarding the surface layer of the etched oxide semiconductor film, and the surface layer is not controlled. Japanese Unexamined Patent Publication 2010-056542 (Patent Document 2) and Japanese Unexamined Patent Publication 2010-056539 (Patent Document 3) also disclose the structure where the interface layer exists in the interface between the oxide semiconductor film and the source/drain electrode. Those Documents disclose the structure where a buffer layer of high carrier density exists in the interface between the source/drain electrode and the IGZO film. The buffer layer is a layer formed with an IGZO film with a high oxygen deficit density and an alloy of the IGZO film and the source-drain metal, which is achieved by being deposited individually. However, nothing is mentioned therein regarding the surface layer of the IGZO film surface.
In the report (Non-Patent Document 2) of C-J. Kim, et al., plasma of an SF6 gas as a fluorine-based gas is used when etching Ti that is a metal material for the source/drain electrode deposited on the upper part of the island-shaped IGZO film. Further, when performing the plasma etching, a substrate is placed on a cathode electrode side. In the oxide semiconductor TFT fabricated in this manner, the off-current is increased since an oxygen deficit layer is formed in the vicinity of the back channel as described above. Thus, it is necessary to remove the oxygen deficit layer by performing etching with hydrochloric acid. Such etching using the hydrochloric acid leads to an increase in the process cost. Also, there is an issue that the yield is dramatically decreased due to over-etching of the IGZO film since selective etching of the Ti electrode and the IGZO film is difficult. Such issues also exist in the case of Kumomi, et al., (Non-Patent Document 3).
As described, there is an issue with the oxide semiconductor TFT that the off-current becomes high since a low-resistance layer due to oxygen depletion tends to be formed on the surface of the oxide semiconductor film. Thus, how to suppress generation of the oxygen deficit layer is an issue.
Further, when the interface layer existing in the interface between the oxide semiconductor film and the source/drain electrode is individually formed as in the cases of the related techniques described above, the process cost is increased. Further, the interface layer is of low resistance, so that it is necessary to remove the interface layer in a part where the oxide semiconductor film and the source/drain electrode are not superimposed on one another by performing etching. However, the compositions of the interface layer and the oxide semiconductor film are similar. Thus, it is difficult to selectively remove only the interface layer by performing etching, so that removal by the etching becomes a cause for deteriorating the yield.
As described above, there is an issue with the oxide semiconductor TFT of the related techniques using the oxide semiconductor film that the off-current becomes high since oxygen depletion is generated in the surface region of the oxide semiconductor film after performing plasma etching of the source/drain electrode. Etching is performed for removing the oxygen deficit layer by using an acid solution such as hydrochloric acid in order to overcome such issue. However, it is a cause for deteriorating the yield and increasing the cost.
The present invention employs following structures in order to overcome such issue.